Keycloak blank page behind nginx reverse proxy

You're nginx configuration looks fine. I recently had the same issue as you had and had more or less the same nginx configuration.

The only thing I still needed to do is to update the standalone configuration file in the keycloak folder. You can find this file at keycloak_folder/standalone/configuration/standalone.xml.

Here you will have to look for the following (+- line 572):

<server name="default-server">
    <http-listener name="default" socket-binding="http" redirect-socket="https" enable-http2="true" />
    ...
</server>

And add the proxy-address-forwarding=true like this:

<server name="default-server">
    <http-listener name="default" socket-binding="http" redirect-socket="https" enable-http2="true" proxy-address-forwarding="true" />
    ...
</server>

This will make sure that your internal keycloak jboss server is aware of the proxy address.